package com.sky.d_lock_free;

import java.lang.reflect.Field;

/** 
 * This class should provide access to low-level operations and its 
 * use should be limited to trusted code.  Fields can be accessed using 
 * memory addresses, with undefined behaviour occurring if invalid memory 
 * addresses are given. 
 * 这个类提供了一个更底层的操作并且应该在受信任的代码中使用。可以通过内存地址 
 * 存取fields,如果给出的内存地址是无效的那么会有一个不确定的运行表现。 
 * 
 * @author Tom Tromey (tromey@redhat.com) 
 * @author Andrew John Hughes (gnu_andrew@member.fsf.org) 
 */  

//Unsafe类源码解析
public class UnsafeParse {
	 // Singleton class.  
    private static UnsafeParse UnsafeParse = new UnsafeParse();  
  
    /** 
     * Private default constructor to prevent creation of an arbitrary 
     * number of instances. 
     * 使用私有默认构造器防止创建多个实例 
     */  
    private UnsafeParse() {  
    }  
  
    /** 
     * Retrieve the singleton instance of <code>UnsafeParse</code>.  The calling 
     * method should guard this instance from untrusted code, as it provides 
     * access to low-level operations such as direct memory access. 
     * 获取<code>UnsafeParse</code>的单例,这个方法调用应该防止在不可信的代码中实例， 
     * 因为UnsafeParse类提供了一个低级别的操作，例如直接内存存取。 
     * 
     * @throws SecurityException if a security manager exists and prevents 
     *                           access to the system properties. 
     *                           如果安全管理器不存在或者禁止访问系统属性 
     */  
    public static UnsafeParse getUnsafeParse() {  
        SecurityManager sm = System.getSecurityManager();  
        if (sm != null)  
            sm.checkPropertiesAccess();  
        return UnsafeParse;  
    }  
  
    /** 
     * Returns the memory address offset of the given static field. 
     * The offset is merely used as a means to access a particular field 
     * in the other methods of this class.  The value is unique to the given 
     * field and the same value should be returned on each subsequent call. 
     * 返回指定静态field的内存地址偏移量,在这个类的其他方法中这个值只是被用作一个访问 
     * 特定field的一个方式。这个值对于 给定的field是唯一的，并且后续对该方法的调用都应该 
     * 返回相同的值。 
     * 
     * @param field the field whose offset should be returned. 
     *              需要返回偏移量的field 
     * @return the offset of the given field. 
     *         指定field的偏移量 
     */  
    public native long objectFieldOffset(Field field);  
  
    /** 
     * Compares the value of the integer field at the specified offset 
     * in the supplied object with the given expected value, and updates 
     * it if they match.  The operation of this method should be atomic, 
     * thus providing an uninterruptible way of updating an integer field. 
     * 在obj的offset位置比较integer field和期望的值，如果相同则更新。这个方法 
     * 的操作应该是原子的，因此提供了一种不可中断的方式更新integer field。 
     * 
     * @param obj    the object containing the field to modify. 
     *               包含要修改field的对象 
     * @param offset the offset of the integer field within <code>obj</code>. 
     *               <code>obj</code>中整型field的偏移量 
     * @param expect the expected value of the field. 
     *               希望field中存在的值 
     * @param update the new value of the field if it equals <code>expect</code>. 
     *               如果期望值expect与field的当前值相同，设置filed的值为这个新值 
     * @return true if the field was changed. 
     *         如果field的值被更改 
     */  
    public native boolean compareAndSwapInt(Object obj, long offset,  
                                            int expect, int update);  
  
    /** 
     * Compares the value of the long field at the specified offset 
     * in the supplied object with the given expected value, and updates 
     * it if they match.  The operation of this method should be atomic, 
     * thus providing an uninterruptible way of updating a long field. 
     * 在obj的offset位置比较long field和期望的值，如果相同则更新。这个方法 
     * 的操作应该是原子的，因此提供了一种不可中断的方式更新long field。 
     * 
     * @param obj    the object containing the field to modify. 
     *               包含要修改field的对象 
     * @param offset the offset of the long field within <code>obj</code>. 
     *               <code>obj</code>中long型field的偏移量 
     * @param expect the expected value of the field. 
     *               希望field中存在的值 
     * @param update the new value of the field if it equals <code>expect</code>. 
     *               如果期望值expect与field的当前值相同，设置filed的值为这个新值 
     * @return true if the field was changed. 
     *         如果field的值被更改 
     */  
    public native boolean compareAndSwapLong(Object obj, long offset,  
                                             long expect, long update);  
  
    /** 
     * Compares the value of the object field at the specified offset 
     * in the supplied object with the given expected value, and updates 
     * it if they match.  The operation of this method should be atomic, 
     * thus providing an uninterruptible way of updating an object field. 
     * 在obj的offset位置比较object field和期望的值，如果相同则更新。这个方法 
     * 的操作应该是原子的，因此提供了一种不可中断的方式更新object field。 
     * 
     * @param obj    the object containing the field to modify. 
     *               包含要修改field的对象 
     * @param offset the offset of the object field within <code>obj</code>. 
     *               <code>obj</code>中object型field的偏移量 
     * @param expect the expected value of the field. 
     *               希望field中存在的值 
     * @param update the new value of the field if it equals <code>expect</code>. 
     *               如果期望值expect与field的当前值相同，设置filed的值为这个新值 
     * @return true if the field was changed. 
     *         如果field的值被更改 
     */  
    public native boolean compareAndSwapObject(Object obj, long offset,  
                                               Object expect, Object update);  
  
    /** 
     * Sets the value of the integer field at the specified offset in the 
     * supplied object to the given value.  This is an ordered or lazy 
     * version of <code>putIntVolatile(Object,long,int)</code>, which 
     * doesn't guarantee the immediate visibility of the change to other 
     * threads.  It is only really useful where the integer field is 
     * <code>volatile</code>, and is thus expected to change unexpectedly. 
     * 设置obj对象中offset偏移地址对应的整型field的值为指定值。这是一个有序或者 
     * 有延迟的<code>putIntVolatile</cdoe>方法，并且不保证值的改变被其他线程立 
     * 即看到。只有在field被<code>volatile</code>修饰并且期望被意外修改的时候 
     * 使用才有用。 
     * 
     * @param obj    the object containing the field to modify. 
     *               包含需要修改field的对象 
     * @param offset the offset of the integer field within <code>obj</code>. 
     *               <code>obj</code>中整型field的偏移量 
     * @param value  the new value of the field. 
     *               field将被设置的新值 
     * @see #putIntVolatile(Object, long, int) 
     */  
    public native void putOrderedInt(Object obj, long offset, int value);  
  
    /** 
     * Sets the value of the long field at the specified offset in the 
     * supplied object to the given value.  This is an ordered or lazy 
     * version of <code>putLongVolatile(Object,long,long)</code>, which 
     * doesn't guarantee the immediate visibility of the change to other 
     * threads.  It is only really useful where the long field is 
     * <code>volatile</code>, and is thus expected to change unexpectedly. 
     * 设置obj对象中offset偏移地址对应的long型field的值为指定值。这是一个有序或者 
     * 有延迟的<code>putLongVolatile</cdoe>方法，并且不保证值的改变被其他线程立 
     * 即看到。只有在field被<code>volatile</code>修饰并且期望被意外修改的时候 
     * 使用才有用。 
     * 
     * @param obj    the object containing the field to modify. 
     *               包含需要修改field的对象 
     * @param offset the offset of the long field within <code>obj</code>. 
     *               <code>obj</code>中long型field的偏移量 
     * @param value  the new value of the field. 
     *               field将被设置的新值 
     * @see #putLongVolatile(Object, long, long) 
     */  
    public native void putOrderedLong(Object obj, long offset, long value);  
  
    /** 
     * Sets the value of the object field at the specified offset in the 
     * supplied object to the given value.  This is an ordered or lazy 
     * version of <code>putObjectVolatile(Object,long,Object)</code>, which 
     * doesn't guarantee the immediate visibility of the change to other 
     * threads.  It is only really useful where the object field is 
     * <code>volatile</code>, and is thus expected to change unexpectedly. 
     * 设置obj对象中offset偏移地址对应的object型field的值为指定值。这是一个有序或者 
     * 有延迟的<code>putObjectVolatile</cdoe>方法，并且不保证值的改变被其他线程立 
     * 即看到。只有在field被<code>volatile</code>修饰并且期望被意外修改的时候 
     * 使用才有用。 
     * 
     * @param obj    the object containing the field to modify. 
     *               包含需要修改field的对象 
     * @param offset the offset of the object field within <code>obj</code>. 
     *               <code>obj</code>中long型field的偏移量 
     * @param value  the new value of the field. 
     *               field将被设置的新值 
     */  
    public native void putOrderedObject(Object obj, long offset, Object value);  
  
    /** 
     * Sets the value of the integer field at the specified offset in the 
     * supplied object to the given value, with volatile store semantics. 
     * 设置obj对象中offset偏移地址对应的整型field的值为指定值。支持volatile store语义 
     * 
     * @param obj    the object containing the field to modify. 
     *               包含需要修改field的对象 
     * @param offset the offset of the integer field within <code>obj</code>. 
     *               <code>obj</code>中整型field的偏移量 
     * @param value  the new value of the field. 
     *               field将被设置的新值 
     */  
    public native void putIntVolatile(Object obj, long offset, int value);  
  
    /** 
     * Retrieves the value of the integer field at the specified offset in the 
     * supplied object with volatile load semantics. 
     * 获取obj对象中offset偏移地址对应的整型field的值,支持volatile load语义。 
     * 
     * @param obj    the object containing the field to read. 
     *               包含需要去读取的field的对象 
     * @param offset the offset of the integer field within <code>obj</code>. 
     *               <code>obj</code>中整型field的偏移量 
     */  
    public native int getIntVolatile(Object obj, long offset);  
  
    /** 
     * Sets the value of the long field at the specified offset in the 
     * supplied object to the given value, with volatile store semantics. 
     * 设置obj对象中offset偏移地址对应的long型field的值为指定值。支持volatile store语义 
     * 
     * @param obj    the object containing the field to modify. 
     *               包含需要修改field的对象 
     * @param offset the offset of the long field within <code>obj</code>. 
     *               <code>obj</code>中long型field的偏移量 
     * @param value  the new value of the field. 
     *               field将被设置的新值 
     * @see #putLong(Object, long, long) 
     */  
    public native void putLongVolatile(Object obj, long offset, long value);  
  
    /** 
     * Sets the value of the long field at the specified offset in the 
     * supplied object to the given value. 
     * 设置obj对象中offset偏移地址对应的long型field的值为指定值。 
     * 
     * @param obj    the object containing the field to modify. 
     *               包含需要修改field的对象 
     * @param offset the offset of the long field within <code>obj</code>. 
     *               <code>obj</code>中long型field的偏移量 
     * @param value  the new value of the field. 
     *               field将被设置的新值 
     * @see #putLongVolatile(Object, long, long) 
     */  
    public native void putLong(Object obj, long offset, long value);  
  
    /** 
     * Retrieves the value of the long field at the specified offset in the 
     * supplied object with volatile load semantics. 
     * 获取obj对象中offset偏移地址对应的long型field的值,支持volatile load语义。 
     * 
     * @param obj    the object containing the field to read. 
     *               包含需要去读取的field的对象 
     * @param offset the offset of the long field within <code>obj</code>. 
     *               <code>obj</code>中long型field的偏移量 
     * @see #getLong(Object, long) 
     */  
    public native long getLongVolatile(Object obj, long offset);  
  
    /** 
     * Retrieves the value of the long field at the specified offset in the 
     * supplied object. 
     * 获取obj对象中offset偏移地址对应的long型field的值 
     * 
     * @param obj    the object containing the field to read. 
     *               包含需要去读取的field的对象 
     * @param offset the offset of the long field within <code>obj</code>. 
     *               <code>obj</code>中long型field的偏移量 
     * @see #getLongVolatile(Object, long) 
     */  
    public native long getLong(Object obj, long offset);  
  
    /** 
     * Sets the value of the object field at the specified offset in the 
     * supplied object to the given value, with volatile store semantics. 
     * 设置obj对象中offset偏移地址对应的object型field的值为指定值。支持volatile store语义 
     * 
     * @param obj    the object containing the field to modify. 
     *               包含需要修改field的对象 
     * @param offset the offset of the object field within <code>obj</code>. 
     *               <code>obj</code>中object型field的偏移量 
     * @param value  the new value of the field. 
     *               field将被设置的新值 
     * @see #putObject(Object, long, Object) 
     */  
    public native void putObjectVolatile(Object obj, long offset, Object value);  
  
    /** 
     * Sets the value of the object field at the specified offset in the 
     * supplied object to the given value. 
     * 设置obj对象中offset偏移地址对应的object型field的值为指定值。 
     * 
     * @param obj    the object containing the field to modify. 
     *               包含需要修改field的对象 
     * @param offset the offset of the object field within <code>obj</code>. 
     *               <code>obj</code>中object型field的偏移量 
     * @param value  the new value of the field. 
     *               field将被设置的新值 
     * @see #putObjectVolatile(Object, long, Object) 
     */  
    public native void putObject(Object obj, long offset, Object value);  
  
    /** 
     * Retrieves the value of the object field at the specified offset in the 
     * supplied object with volatile load semantics. 
     * 获取obj对象中offset偏移地址对应的object型field的值,支持volatile load语义。 
     * 
     * @param obj    the object containing the field to read. 
     *               包含需要去读取的field的对象 
     * @param offset the offset of the object field within <code>obj</code>. 
     *               <code>obj</code>中object型field的偏移量 
     */  
    public native Object getObjectVolatile(Object obj, long offset);  
  
    /** 
     * Returns the offset of the first element for a given array class. 
     * To access elements of the array class, this value may be used along with 
     * with that returned by 
     * <a href="#arrayIndexScale"><code>arrayIndexScale</code></a>, 
     * if non-zero. 
     * 获取给定数组中第一个元素的偏移地址。 
     * 为了存取数组中的元素，这个偏移地址与<a href="#arrayIndexScale"><code>arrayIndexScale 
     * </code></a>方法的非0返回值一起被使用。 
     * 
     * @param arrayClass the class for which the first element's address should 
     *                   be obtained. 
     *                   第一个元素地址被获取的class 
     * @return the offset of the first element of the array class. 
     *         数组第一个元素 的偏移地址 
     * @see arrayIndexScale(Class) 
     */  
    public native int arrayBaseOffset(Class arrayClass);  
  
    /** 
     * Returns the scale factor used for addressing elements of the supplied 
     * array class.  Where a suitable scale factor can not be returned (e.g. 
     * for primitive types), zero should be returned.  The returned value 
     * can be used with 
     * <a href="#arrayBaseOffset"><code>arrayBaseOffset</code></a> 
     * to access elements of the class. 
     * 获取用户给定数组寻址的换算因子.一个合适的换算因子不能返回的时候(例如：基本类型), 
     * 返回0.这个返回值能够与<a href="#arrayBaseOffset"><code>arrayBaseOffset</code> 
     * </a>一起使用去存取这个数组class中的元素 
     * 
     * @param arrayClass the class whose scale factor should be returned. 
     * @return the scale factor, or zero if not supported for this array class. 
     */  
    public native int arrayIndexScale(Class arrayClass);  
  
    /** 
     * Releases the block on a thread created by 
     * <a href="#park"><code>park</code></a>.  This method can also be used 
     * to terminate a blockage caused by a prior call to <code>park</code>. 
     * This operation is UnsafeParse, as the thread must be guaranteed to be 
     * live.  This is true of Java, but not native code. 
     * 释放被<a href="#park"><code>park</code></a>创建的在一个线程上的阻塞.这个 
     * 方法也可以被使用来终止一个先前调用<code>park</code>导致的阻塞. 
     * 这个操作操作时不安全的,因此线程必须保证是活的.这是java代码不是native代码。 
     * 
     * @param thread the thread to unblock. 
     *               要解除阻塞的线程 
     */  
    public native void unpark(Thread thread);  
  
    /** 
     * Blocks the thread until a matching 
     * <a href="#unpark"><code>unpark</code></a> occurs, the thread is 
     * interrupted or the optional timeout expires.  If an <code>unpark</code> 
     * call has already occurred, this also counts.  A timeout value of zero 
     * is defined as no timeout.  When <code>isAbsolute</code> is 
     * <code>true</code>, the timeout is in milliseconds relative to the 
     * epoch.  Otherwise, the value is the number of nanoseconds which must 
     * occur before timeout.  This call may also return spuriously (i.e. 
     * for no apparent reason). 
     * 阻塞一个线程直到<a href="#unpark"><code>unpark</code></a>出现、线程 
     * 被中断或者timeout时间到期。如果一个<code>unpark</code>调用已经出现了， 
     * 这里只计数。timeout为0表示永不过期.当<code>isAbsolute</code>为true时， 
     * timeout是相对于新纪元之后的毫秒。否则这个值就是超时前的纳秒数。这个方法执行时 
     * 也可能不合理地返回(没有具体原因) 
     * 
     * @param isAbsolute true if the timeout is specified in milliseconds from 
     *                   the epoch. 
     *                   如果为true timeout的值是一个相对于新纪元之后的毫秒数 
     * @param time       either the number of nanoseconds to wait, or a time in 
     *                   milliseconds from the epoch to wait for. 
     *                   可以是一个要等待的纳秒数，或者是一个相对于新纪元之后的毫秒数直到 
     *                   到达这个时间点 
     */  
    public native void park(boolean isAbsolute, long time);  
}
